Among the more invidious aspects of cancer is how it hijacks an organism's own cells and propagates rapidly while remaining intermingled with healthy tissue. For this reason, scientists and engineers have found it difficult to develop a cancer treatment that can distinguish between healthy tissue that should be left alone and cancerous cells that must be destroyed. For example, radiation therapy is used against cancer because the rapidly growing cancer cells divide faster and are therefore more susceptible to radiation. However, all living cells in a patient's body are continuously dividing so the radiation also causes harm to healthy tissue and in turn causes the well known debilitating side effects of radiation therapy. Given that this aspect of cancer is a chief contributor to its status as a worldwide epidemic, a broad tranche of cancer research is directed to the production of treatments that can be more accurately targeted to the cancer itself.
The use of accelerated protons to bombard cancer cells was pioneered in the middle of the 20th century by nuclear scientists working in particle accelerator laboratories. Since then the field of proton therapy has developed into a successful weapon in the medical profession's arsenal against cancer. The general concept of proton therapy involves bombarding a tumor using a beam of accelerated protons. As with other types of radiotherapy the protons are a form of ionizing radiation that more strongly effect cells that are rapidly dividing. In addition, the beam can be focused directly on a tumor and will therefore cause minimal harm to the surrounding healthy tissue. In this sense proton therapy is similar to other forms of beam directed radiation treatments such as x-ray radiation therapy. However, proton therapy combined with the spot scanning carries the additional and unique benefit of better three dimensional dose conformity than known therapy methods on the market. In most cases no patient specific collimators are needed.
As seen in FIG. 1, proton therapy is superior to x-ray radiation therapy in terms of its ability to prevent damage to surrounding healthy tissue. X-axis 101 shows the depth of the particles and y-axis 102 shows the proportional radiation dose delivered at a given depth. The proportional dose of radiation delivered by the photons in x-ray radiation therapy is shown by photon dose distribution line 103. Photon dose distribution line 103 peaks at a low depth and then gradually tapers out. To increase the radiation delivered at a desired depth, damage to the healthy tissue above the tumor must be proportionally increased. In comparison, proton dose distribution line 104 minimizes the radiation delivered before and after the target and delivers nearly all of its energy in a given window of depth. The peak of the proton dose distribution line is called the Bragg peak.
The acceleration of protons requires the use of a particle accelerator. Two common types of particle accelerators are cyclotrons and synchrotrons. Both types of accelerators depend on the interplay of magnetic and electric fields. Synchrotrons accelerate particles through a path having a constant radius and adjust the magnetic and electric fields as the particles gain momentum. Cyclotrons accelerate charged particles using a high frequency alternating voltage. A perpendicular magnetic field causes the particles to move in an expanding spiral wherein they re-encounter the accelerating voltage. When the particles reach a predetermined radius they are guided out of the cyclotron in an accelerated state.